xfs: remove the xfs_ifork_t typedef

We only have a few more callers left, so seize the opportunity and kill
it off.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>